Piezoelectric/electrostrictive film elements of this kind have conventionally been used as the above-mentioned actuators and sensors. The conventional configuration of such a piezoelectric/electrostrictive film element has been disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 5-267742, 6-260694, and 8-201265.
A piezoelectric/electrostrictive film element used as a sensor is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 8-201265. The piezoelectric/electrostrictive film sensor is used to measure fluid characteristics, such as density, concentration, and viscosity. The piezoelectric/electrostrictive film sensor is configured to utilize the interrelation between the amplitude of a piezoelectric/electrostrictive film vibrator and viscous drag of fluid in contact with the vibrator for measuring density, concentration, viscosity, etc. of the fluid.
Specifically, in the piezoelectric/electrostrictive film sensor, when the vibrator is vibrated in the presence of the fluid, the vibrator is subjected to a dynamic resistance caused by viscosity of the fluid. A mechanical vibrating condition of the vibrator can be replaced with an equivalent electrical circuit. On the basis of viscous drag to which the vibrator is subjected, an electrical constant of an equivalent circuit of a piezoelectric/electrostrictive film used to constitute the vibrator varies. By detecting variation of the electrical constant, characteristics, such as viscosity, density, and concentration, of the fluid can be measured.
Fluid whose characteristic can be measured by such a piezoelectric/electrostrictive film sensor encompasses liquid and gas. Examples of liquid which serves as a subject of measurement include, as a matter of course, a single-component liquid formed solely of a main medium selected from among, for example, water, alcohol, oil, etc. Also, the examples of the subject-of-measurement liquid can include a liquid substance (slurry, paste, or the like) formed in such a manner that another medium that is soluble, lightly soluble, or insoluble in such a main medium is added to (dissolved in, mixed with, diffused in, or suspended in) the main medium.
Also, examples of the above-mentioned electrical constant include loss factor, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. Particularly, phase, or loss factor, which has a single maximal or minimal variation point in the vicinity of the resonance frequency of an equivalent circuit is preferably used. This enables measurement of density and concentration of the fluid in addition to viscosity of the fluid. For example, the concentration of sulfuric acid in an aqueous solution of sulfuric acid can be measured. In addition to the above-mentioned electrical constants, variation of resonance frequency can be used as an index for detecting variation of vibrating condition, so long as no particular problem arises in terms of accuracy of measurement, and durability.